I found an easier way to beat Hannah:
What you need:
- Approx. 30 Super Potions ($700 each) = $21,000
- 3-4 untrained Geodudes or other Pokemon with Sturdy ability and Toxic attack which have not more than 51 Max. HP (less of these Pokemon can work too, but you never know how often Toxic fails, your Geodude flinches or Hannah's Pokemon faints before you can heal your Geodude with a Super Potion)
- 1-2 Tank Pokemon to defeat Mega Metagross, because Toxic has no effect on it: E.g. 1x Lv 100 Slowbro holding Leftovers with Shadowball attack (EVs: 252 on HP and SpAtk)
- Untrained Huntail
How to battle:
- Defeat Hannah's Houndoom, Machamp, Talonflame, Volcarona and Mega Sharpedo with your Geodudes by attacking with Toxic as first move and using Super Potion when your Pokemon's HP aren't full.
- To defeat Mega Metagross, switch to Slowbro and just use Shadowball until it faints. Alternatively you can use another 1-2 Pokemon that can defeat it.